Marvel - and Disney - are movie-making machines. They churn out movie after movie to stay on top of the film industry. But machine-made products in any industry tend to all look the same. And the same goes for all the movies in the MCU. All of Marvel's movies follow the same basic formula. Their plots always loosely follow Joseph Campbell's "hero's journey:" the call to adventure, meeting a mentor, conflict with the foe, a "darkest hour," and eventual triumph. But there are also other similarities specific to Marvel movies. There's always an "emotionally-distant antihero," a dead mentor, a token "strong female character," an internal pep-talk revelation, and an anticlimactic final battle. Look past some of the specific aesthetics and character names and you'll see the same story told over and over again. Sure, it might be an entertaining watch in itself, the first time around. But such a surface-level hero story told over and over again just wears on an audience. It certainly doesn't merit praise as good cinema.
There are certain story beats common to the Marvel movies. But anyone taking a cursory glance at the plots of these movies can see that the differences are quite substantial. Does Guardians of the Galaxy, a space action-comedy about surrogate family, really resemble Black Panther, a story about political competition and oppression? Is Captain America: The First Avenger, about Steve Rogers gaining the strength to fight for his country, really the same as his succeeding films in which he becomes disillusioned with his country and fights against it? There is much more variety in the MCU than critics give it credit for.